About 50 companies were affected by a major security breach at e-mail service provider Epsilon Interactive that caused many U.S. corporations to warn their customers of online attacks Monday.

Epsilon first warned of the incident Friday, saying that someone had got into company systems and obtained e-mail addresses and names belonging to some of its customers. But it wasn't immediately clear how many of its 2,500 clients were at risk. Epsilon is still being tight-lipped about the problem, but it has now given a clearer picture of how many companies are affected.

Customers of many of these businesses, which include Target, Citigroup, Tivo and Walgreens, woke up Monday to e-mail warnings, telling them that their e-mail addresses had been stolen, and that spam or malicious messages could be coming their way. But so far, Epsilon has refused to provide a detailed list of all companies that were affected.

Companies hire Epsilon to send out a total of more than 40 billion messages on their behalf each year.

With millions of addresses thought to have been stolen, the problem may be worse than many people realize, security experts said Monday.

That's because once scammers know their victims' names and e-mail addresses, along with the companies that they do business with, they can craft very targeted "spear-phishing" e-mail attacks that try to trick victims into revealing more sensitive information such as passwords or account numbers.

"Everybody is downplaying it by saying, 'at least they didn't get financial information.' Well that's true, but what they did get was enough to potentially get financial information [in a phishing attack]," said Neil Schwartzman, executive director with the Coalition Against Unsolicited Commercial Email, a consumer advocacy group based in Montreal.

The IDG News Service has confirmed that the following companies have warned customers about the breach. Because Epsilon is often engaged by business units within these firms, not everyone who does business with these companies has had their e-mail address stolen. If you have received a notification from a company that is not included on this list, please contact robert_mcmillan@idg.com.

AbeBooks

Ameriprise Financial

Barclays Bank

Best Buy

Brookstone

Citibank

Disney Destinations

Hilton Worldwide

JP Morgan Chase

Kroger

Lacoste

Marriott International

McKinsey Quarterly

New York & Company

Robert Half

Target

Tivo

US Bank

Walgreens

Robert McMillan covers computer security and general technology breaking news for The IDG News Service. Follow Robert on Twitter at @bobmcmillan. Robert's e-mail address is robert_mcmillan@idg.com